The present invention relates generally to desalination of seawater, and is particularly concerned with clathrate freeze desalination. It also relates to the combination of clathrate freeze desalination of seawater and air conditioning.
Various techniques have been proposed in the past for obtaining fresh water from seawater, but most of these have been higher cost processes that had limited commercial feasibility. Historically, the original methods proposed for desalinating seawater involved distillation, where seawater is heated to the boiling point and the water vapor released is condensed as fresh water. Later, reverse osmosis, involving the diffusion of fresh water from seawater through a semi-permeable membrane, was also developed for use where higher water prices were acceptable. Concurrently, various processes were proposed for desalinating seawater by freezing. These processes, to the extent developed at that time, all proved to be too expensive for commercial use. Some of these processes involved indirect freezing, in which freezing is accomplished by circulating a cold refrigerant through a heat exchanger to remove heat from the seawater. Ice is formed on the heat exchanger surface and must be removed, washed and melted to produce fresh water.
Another category of freeze desalination is by direct freezing, in which heat is removed from seawater by direct contact with a refrigerant, which may be seawater itself, in a vacuum freezing vapor compression process, or alternatively, by use of a secondary refrigerant. In the latter process, a refrigerant (that has a low solubility in water) is compressed, cooled to a temperature close to the freezing temperature of salt water, and mixed with seawater. As the refrigerant evaporates, heat is absorbed from the mixture and the water freezes into ice. Butane is a possible secondary refrigerant for such a process.
Another type of direct freezing desalination process is called gas hydrate or clathrate freeze desalination. This process involves the use of a class of agents that form gas hydrates, or clathrates, with water at temperatures higher than the normal freezing temperature of water. A clathrate is an aggregation of water molecules around a central hydrocarbon, or other non-water molecule, to form an ice crystal. When clathrate "ice" is melted, fresh water and the clathrate forming agent are recovered, thus producing fresh water and regenerating the clathrate forming agent simultaneously. This has an advantage over other direct freezing processes in that the operating temperature is higher, reducing power requirements to both form and melt the "ice." Various alternative proposals for freeze desalination are described in a paper entitled, "Desalination by Freezing" by Herbert Wiegandt, School of Chemical Engineering, Cornell University, March 1990. Several demonstration plants for conducting freeze desalination feasibility and economic testing were designed and constructed by the U.S. Department of the Interior, Office of Saline Water, from 1955 through 1974. However, these were discontinued due to lack of funds and to problems encountered in their operation. In spite of considerable research on clathrate freeze desalination for a number of years, it was not considered to be a commercially viable alternative, due to technical problems and high operating costs. Test plants built for clathrate freeze desalination did not meet design criteria, mainly because the hydrate crystals were very small and both difficult and expensive to separate from the brine.